This invention relates generally to a epicyclic gear arrangement for transmitting and combining torque from a plurality of sources or parallel power paths, and more particularly to a helicopter gearbox having redundant gearing.
In helicopters, power is commonly supplied by a pair of turbine engines running at 20,000 rpm or more. This power must be combined and delivered to a rotor at a speed that is reduced from the turbine speed by a factor of 50 to 150. It is desirable to split the drive train so that power is delivered within the transmission elements by two or more routes. In this way gear tooth loading is reduced, and operating safety is improved by the redundant gearing should one gear set fail during flight. Split torque gearing is known; for example, such arrangement of gears is disclosed in U.S. Pats. Nos. 5,135,442 and 5,802,918, the teachings of which are entirely incorporated herein by reference.
In the present invention, the main rotor of a helicopter is driven by an epicyclic gearbox comprising a rotor sun gear coaxial and co-rotational with the rotor, a stationary sun gear coaxial with the rotor sun gear, at least one planet having an axis parallel to and rotating about the sun gear axis, and making contact with both sun gears. The planet comprises first and second planetary coaxial gears mechanically constrained to rotate together. The first planetary gear is in meshing engagement with the stationary sun gear and the second planetary gear is in meshing engagement with the rotor sun gear. The first planetary gear is also in meshing engagement with the internal teeth of a first ring gear and the second planetary gear is also in meshing engagement with the internal teeth of a second ring gear. The first and second ring gears each have a second set of gear teeth (bull teeth) on the top, bottom or sides thereof for meshing engagement with one or more drive pinions. These bull teeth and may be external, face, or even internal. In a first embodiment, a first drive pinion drives the first ring gear and a second drive pinion drives the second ring gear in the same direction. Power derives from individual shaft connection with a pair of engines. In a second embodiment of the invention, input shaft power is further split prior to delivery to the epicyclic gearbox by a plurality of drive pinions.
The first planetary gear has a first planetary pitch diameter, and the second planetary gear has a second planetary pitch diameter, wherein the first planetary pitch diameter is different from the second planetary pitch diameter.
The present invention provides a plurality of power paths into the epicyclic gearing thereby reducing tooth loading and increasing the operational safety factor of the aircraft by adding drive redundancy.
It is therefore an object of the present invention to provide a epicyclic gearbox having parallel power input paths combined by at least one planetary gear.
It is another object of at least one embodiment of the invention to provide a gearbox having a plurality of power paths for driving the main rotor of a helicopter.